Vehicle-to-vehicle distance calculation apparatus and method

ABSTRACT

The distance to a target vehicle is calculated comparatively accurately. To achieve this, a target vehicle traveling ahead of one&#39;s own vehicle is imaged by a camera and it is determined to what vehicle group, such as a light-duty vehicle group, standard passenger car group or heavy-duty vehicle group, the image of the target vehicle belongs. A first distance from one&#39;s own vehicle to the target vehicle is calculated by a circuit using the representative vehicle width of the vehicle group decided. A vanishing point is detected from the captured image by a vanishing point detection circuit and a second distance from one&#39;s own vehicle to the target vehicle is calculated utilizing the vanishing point. The distance to the target vehicle is decided from the first and second distances by a distance decision circuit, wherein the shorter the distance to the vanishing point, the more the value of a weighting coefficient of the second distance is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a vehicle-to-vehicle distance calculationapparatus and method.

2. Description of the Related Art

Calculation of vehicle-to-vehicle distance is important in order toprevent vehicular accidents. In order to achieve this, there is a systemthat detects the shadow of the vehicle traveling ahead of one's ownvehicle and calculates vehicle-to-vehicle distance using the position ofthe shadow and a vanishing point (Patent Document 1). Further, there isa system that detects an obstacle by utilizing a vanishing point (PatentDocument 2) and a system that detects whether an object is a moving bodyby utilizing a vanishing point (Patent Document 3).

[Patent Document 1]: Japanese Patent Application Laid-Open No.2002-327635

[Patent Document 2]: Japanese Patent Application Laid-Open No.2007-199932

[Patent Document 3]: Japanese Patent Application Laid-Open No.2006-48338

However, since the position of the shadow of the vehicle traveling aheadmust be detected in Patent Document 1, the system is comparativelycomplex and cost of development is high. In addition, there areinstances where it takes too much time to calculate distance.Furthermore, since it is difficult to detect a shadow at night or on asnow-covered road, etc., there are instances where distance cannot becalculated. Further, in both Patent Documents 1 and 2, no considerationis given to calculation of vehicle-to-vehicle distance in acomparatively simple and accurate manner.

SUMMARY OF THE INVENTION

An object of the present invention is to calculate vehicle-to-vehicledistance comparatively simply and accurately.

A vehicle-to-vehicle distance calculation apparatus according to thepresent invention comprises: an imaging control device (imaging controlmeans) for controlling a camera, which has been mounted on one's ownvehicle, so as to image a target vehicle present ahead of one's ownvehicle; a vehicle group decision device (vehicle group decision means)for deciding to what group the target vehicle belongs based upon animage obtained by imaging by the camera; a target vehicle pixel countcalculation device (target vehicle pixel count calculation means) forcalculating number of width or height pixels of a target image containedin an image obtained by imaging by the camera; a first distancecalculation device (first calculation means) for calculating thedistance to the target vehicle based upon a representative vehicle widthor vehicle height of the vehicle group decided by the vehicle groupdecision device, the number of pixels calculated by the target vehiclepixel count calculation device and the number of width or height pixelsof the image obtained by imaging by the camera; a target-vehicle imagedetection device (target-vehicle image detection means) for detecting atarget-vehicle image, which represents a target vehicle, from within theimage obtained by imaging by the camera; a vanishing point detectiondevice (vanishing point detection means) for detecting a vanishing pointfrom within the image obtained by imaging by the camera; a seconddistance calculation device (second distance calculation means) forcalculating the distance to the target vehicle based upon a position ofat least one of an upper edge and lower edge of the target-vehicle imagedetected by the target-vehicle image detection device and position ofthe vanishing point detected by the vanishing point detection device;and a distance decision device (distance decision means) for decidingthe distance to the target vehicle from the first distance calculated bythe first distance calculation device and the second distance calculatedby the second distance calculation device upon applying weighting suchthat the closer together the position of the target-vehicle imagedetected by the target-vehicle image detection device and position ofthe vanishing point detected by the vanishing point detection device,the smaller the weight of the second distance calculated by the seconddistance calculation device becomes.

The invention also provides a vehicle-to-vehicle distance calculationmethod. Specifically, the invention provides a method of calculatingvehicle-to-vehicle distance, comprising steps of: controlling a camera,which has been mounted on one's own vehicle, so as to image a targetvehicle present ahead of one's own vehicle; deciding to what group thetarget vehicle belongs based upon an image obtained by imaging by thecamera; calculating number of width or height pixels of a target imagecontained in an image obtained by imaging by the camera; calculating thedistance to the target vehicle based upon a representative vehicle widthor vehicle height of the vehicle group decided, the number of pixelscalculated and the number of width or height pixels of the imageobtained by imaging by the camera; detecting a target-vehicle image,which represents a target vehicle, from within the image obtained byimaging by the camera; detecting a vanishing point from within the imageobtained by imaging by the camera; calculating the distance to thetarget vehicle based upon a position of at least one of an upper edgeand lower edge of the target-vehicle image detected and position of thevanishing point detected; and deciding the distance to the targetvehicle from the first distance calculated and the second distancecalculated upon applying weighting such that the closer together theposition of the target-vehicle image detected and position of thevanishing point detected, the smaller the weight of the second distancecalculated becomes.

In accordance with the present invention, a target vehicle located aheadof one's own vehicle is imaged and to what vehicle group the targetvehicle belongs is decided from the image obtained by imaging. Further,the number of width or height pixels of the image of the target vehiclecontained in the image obtained by imaging is calculated. The distancefrom one's own vehicle to the target vehicle is calculated based upon arepresentative vehicle width or vehicle height of the vehicle groupdecided, the calculated number of width or height pixels of the targetvehicle and the number of width or height pixels of the image obtainedby imaging. For example, a vehicle group is a group classified accordingto the width or height of a vehicle such as a light-duty automobile,standard passenger car, heavy-duty vehicle such as a truck or bus,motorcycle or bicycle and can be said to correspond to the class ofdriver's license. In accordance with the present invention, to whatvehicle group a target vehicle belongs is decided and a first distanceto the target vehicle is calculated utilizing the representative vehiclewidth or height of the vehicle group decided.

Furthermore, in an accordance with the present invention, the targetvehicle ahead of one's own vehicle is imaged. A second distance to thetarget vehicle is calculated based upon a position of at least one of anupper edge and lower edge of a target-vehicle image, which representsthe target vehicle from within the captured image, and the position ofthe vanishing point.

The distance to the target vehicle is decided from the calculated firstand second distances in such a manner that the shorter the distance fromthe target-vehicle image to the vanishing point, the smaller the weightof the second distance. In a case where the target-vehicle image existsnear the vanishing point, any error in detection of the vanishing pointexerts a large influence upon the calculation of the second distance. Inaccordance with the present invention, however, if the target-vehicleimage exists near the vanishing point, the distance to the targetvehicle is decided from the first and second distances upon lowering theweighting of the second distance.

The apparatus may further comprise a vehicle memory in whichrepresentative vehicle widths or vehicle heights have been storedbeforehand on a per-vehicle-group basis. In this case, the firstdistance calculation device would calculate the distance to the targetvehicle based upon a representative vehicle width or vehicle height ofthe vehicle group, which has been decided by the vehicle group decisiondevice, from among the representative vehicle widths or vehicle heightsthat have been stored in the vehicle memory, the number of pixelscalculated by the target vehicle pixel count calculation device, and thenumber of width or height pixels of the image obtained by imaging by thecamera.

The first distance calculation device may include a third distancecalculation device for calculating a third distance to the targetvehicle based upon the upper edge of the target-vehicle image detectedby the target-vehicle image detection device and the position of thevanishing point detected by the vanishing point detection device; and afourth distance calculation device for calculating a fourth distance tothe target vehicle based upon the lower edge of the target-vehicle imagedetected by the target-vehicle image detection device and the positionof the vanishing point detected by the vanishing point detection device.In this case, the distance to the target vehicle would be calculatedbased upon the third distance calculated by the third distancecalculation device and the fourth distance calculated by the fourthdistance calculation device.

The apparatus may further comprise: a determination device(determination means) for determining whether a tire of the targetvehicle is absent below the lower edge of the target vehicle detected bythe target-vehicle image detection device; and a correction device forcorrecting the position of the lower edge of the target vehicle, whichhas been detected by the target-vehicle image detection device, inresponse to a determination by the determination device that a tire isabsent. In this case, the second distance calculation device wouldcalculate the distance to the target vehicle using the position of thelower edge corrected by the correction device.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the relationship between one's own vehicle and atarget vehicle;

FIG. 2 is a block diagram illustrating the electrical configuration of avehicle-to-vehicle distance calculation apparatus;

FIG. 3 is an example of an image obtained by imaging;

FIG. 4 is an example of a vehicle group table;

FIGS. 5A and 5B illustrate relationships between one's own vehicle and atarget vehicle;

FIGS. 6 and 7 are examples of images obtained by imaging;

FIGS. 8 and 9 are examples of target-vehicle images; and

FIG. 10 illustrates the relationship between one's own vehicle and atarget vehicle;

FIG. 11 illustrates the relationship between distance to a vanishingpoint and weighting coefficients; and

FIGS. 12 and 13 illustrate driving tendencies.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 represents in side view the relationship between one's ownvehicle 2 and a target vehicle 1 traveling ahead of the vehicle 2.

One's own vehicle (an automotive vehicle) 2 is traveling on a road 3 andthe target vehicle (an automotive vehicle) 1, whose vehicle-to-vehicledistance d is to be calculated, is traveling ahead of one's own vehicle2.

A camera 10 is mounted within one's own vehicle 2 at the forward endnear the top of the vehicle at a position having a height h. The targetvehicle traveling ahead of one's own vehicle 2 is imaged by the camera10. Vehicle-to-vehicle distance d from one's own vehicle 2 to the targetvehicle 1 is calculated based upon the image captured by the camera 10.The distance from the position at which the camera 10 is mounted to thefront end of one's own vehicle 2 is Δd.

FIG. 2 is a block diagram illustrating the electrical configuration of avehicle-to-vehicle distance calculation apparatus.

In this embodiment, first and second distance calculations areperformed. In the first distance calculation, the vehicle group(light-duty vehicle, standard passenger car, heavy-duty vehicle) of thetarget vehicle is decided and the distance to the distance to the targetvehicle 1 is calculated utilizing the vehicle group. In the seconddistance calculation, a vanishing point is detected and the distance tothe distance to the target vehicle 1 is calculated utilizing thevanishing point. The vehicle-to-vehicle distance d to the target vehicleis calculated based upon a first vehicle-to-vehicle distance d1 obtainedin the first distance calculation and a second vehicle-to-vehicledistance d2 obtained in the second distance calculation.

The first distance calculation will be described first.

The overall operation of the vehicle-to-vehicle distance calculationapparatus is controlled by a control unit 20.

The camera 10 is controlled by an imaging control unit 23. The targetvehicle 1 traveling (stopped) ahead of one's own vehicle 2 is imaged bythe camera 10.

FIG. 3 is an example of an image 30 obtained by imaging.

In the image 30, a target-vehicle image 1 (indicated by the samereference numeral as that of the target vehicle 1) of the target vehicle1 traveling ahead is represented on a road image 3 (indicated by thesame reference numeral as that of the road 3) representing the road 3.The target-vehicle image 1 includes also an image 1A of a license plate.An image 4 of a roadway boundary block and an image 5 of the center lineof the road are shown bracketing the target-vehicle image 1.

As will be described later, vehicle detection processing is executedbased upon the image 30 to thereby detect the target-vehicle image 1.Also displayed in FIG. 3 is a vehicle frame 40 indicating the fact thatthe target-vehicle image 1 has been detected.

With reference again to FIG. 2, image data representing the image 30captured by the camera 10 is input to a vehicle image detection circuit11. In the vehicle image detection circuit 11, the target-vehicle image1 is detected from within the image 30 as mentioned above. Datarepresenting the detected target-vehicle image 1 is applied to a vehiclegroup decision circuit 12.

The vehicle group decision circuit 12 detects the vehicle group of thetarget vehicle 1, which is traveling ahead of one's own vehicle 2, fromthe entered data representing the target-vehicle image 1. (Since thevehicle group indicates the class of vehicle corresponding to vehiclesize, in this embodiment groups are classified into light-dutyautomobiles, standard passenger cars, heavy-duty vehicles such as truckor bus, motorcycles or bicycles.) When the vehicle group is decided, arepresentative width w of this vehicle group is determined by referringto FIG. 4.

FIG. 4 is an example of a vehicle group memory contained in a vehiclewidth memory 13.

Representative vehicle widths of vehicle groups have been stored in thevehicle width memory in correspondence with the vehicle groups. It goeswithout saying that the vehicle groups that have been stored in thevehicle group memory correspond to vehicle groups that can be decided inthe vehicle group decision circuit 12. Vehicle widths w1, w2, w3, w4 andw5 have been stored in correspondence with the vehicle groups, namelylight-duty automobile, standard passenger car, heavy-duty vehicle (truckor bus), motorcycle and bicycle.

When the data representing the vehicle group decided in the vehiclegroup decision circuit 12 is input to the vehicle width memory 13, thedata representing the representative vehicle width corresponding to thisvehicle group is output from the vehicle width memory 13. The datarepresenting the vehicle width is applied to a first distancecalculation circuit 14.

When the vehicle group is decided, a number Na of pixels indicative ofthe width of the bottom side of the target-vehicle image 1 (see FIG. 3)is calculated as well.

FIG. 5A is a plan view illustrating the positional relationship betweenone's own vehicle 2 and the target vehicle 1 at a certain time t1.

In FIG. 5A, a view angle θ of the camera 10 has been decided and anumber Nw of pixels indicative of the width of the output image 30 alsohas been decided. If we let x1 represent the actual width capable ofbeing imaged by the camera at the position of the rear end oftarget-vehicle image 1 and let d10 represent the distance from thetarget vehicle 1 to one's own vehicle 2, then Equation 1 below willhold.

d10=x1/[2 tan(θ/2)]  Equation 1

Further, since Nw:x1=Na:w holds, we have Equation 2 below.

x1=Nw×w/Na  Equation 2

Equation 3 below is obtained from Equations 1 and 2, andvehicle-to-vehicle distance d10 can be calculated from Equation 3.

d10=Nw×w/[2×Na×tan(θ/2)]  Equation 3

FIG. 5B is a plan view illustrating the positional relationship betweenone's own vehicle 2 and the target vehicle 1 at a time t2 reached uponelapse of a unit time from time t.

If we let d20 represent the vehicle-to-vehicle distance from one's ownvehicle 2 to the target vehicle 1, then the vehicle-to-vehicle distanced20 can be calculated from Equation 4 below at time t2 in the manner setforth above.

d20=Nw×w/[2×Na×tan(θ/2)]  Equation 4

The vehicle-to-vehicle distances d10 and d20 represent the firstdistance d1. Data representing the first distance d1 decided as setforth above is applied to a distance decision circuit 15.

The second distance calculation will be described next.

FIG. 6 is an example of image 30 obtained by imaging by the camera 10.

The image 30 contains a road image 3 (indicated by the same referencenumeral as that of the road 3) representing the road 3 of the lane alongwhich one's own vehicle 2 travels, and a road image 3A of the lane alongwhich oncoming vehicles travel. An image 5 of the center line of theroad is displayed between the road image 3 of one's own traveling laneand the road image 3A of the traveling lane of oncoming vehicles.Further, an image 4 of a roadway boundary block is displayed on the leftside of the road image 3 and on the right side of the road image 3A.

In the second distance calculation, a vanishing point Pv is utilized inorder to measure the distance to the target vehicle 1.

The vanishing point Pv is located at a position where an extension ofthe roadway boundary block 4 intersects an extension of the center line5. In a case where either one of these cannot be found, the vanishingpoint Pv may be obtained by the position where either one of theseintersects these parallel lines (e.g., an extension of a guard rail).

FIG. 7 is an example of an image 30A obtained by imaging.

The image 30A contains a target-vehicle image 1 representing the targetvehicle 1 that travels ahead of one's own vehicle 2. A frame 40 thatspecifies the target-vehicle image 1 detected from the image 30A also isdisplayed surrounding the target-vehicle image 1.

A Y-coordinate (a coordinate along the vertical direction) position ybof the lower end of the target-vehicle image 1 is detected, and aY-coordinate position ye of the vanishing point Pv is detected. Thedistance (second distance) d2 to the target vehicle 1 is calculatedusing the difference Δy between the detected positions yb and ye.

With reference again to FIG. 2, when what is ahead of one's own vehicle2 is imaged by the camera 10, the image data representing the capturedimage is input to a vehicle image detection circuit 11. In the vehicleimage detection circuit 11, the target-vehicle image 1 is detected fromwithin the image 30 as mentioned above. Data representing the detectedtarget-vehicle image 1 is applied to a lower edge position decisioncircuit 26.

The lower edge position decision circuit 26 detects the position yb ofthe lower edge of the target vehicle (target-vehicle image 1) travelingahead of one's own vehicle 2. Data representing the detected position ybis input to a lower edge position correction circuit 27. The lower edgeposition correction circuit 27 corrects the detected lower edge positionyb. The details concerning this correction processing will be describedlater. Data representing the lower-edge position corrected in the loweredge position correction circuit 27 is input to a second distancecalculation circuit 29.

Further, data representing the image captured by the camera 10 is inputto a vanishing point detection circuit 28 as well. The vanishing pointdetection circuit 28 detects the vanishing point from the capturedimage. Data indicating the position ye of the detected vanishing pointalso is input to the second distance calculation circuit 29.

The second distance calculation circuit 29 calculates the distance(second distance) d2 to the target vehicle 1 utilizing data such as theentered data indicating the vanishing-point position ye and dataindicating the lower-edge position yb of the target vehicle 1.

With reference to FIG. 1, and as mentioned above, let d represent thevehicle-to-vehicle distance (second distance d2), let Δd represent thedistance from the position at which the camera 10 is mounted on one'sown vehicle 2 to the front end of one's own vehicle 2, and let hrepresent the height at which the camera 10 is mounted. Further, let Δh1represent the height of the lower edge of the rear end of target vehicle1 from the road 3.

With reference to FIG. 7, and as mentioned above, ye represents theY-coordinate position of the vanishing point Pv in the image 30Aobtained by imaging, and yb represents the Y-coordinate position of thelower edge of the rear end of the detected target-vehicle image 1.Further, Δy represents the distance between the Y-coordinate positionsye and yb.

If we let Δθ (rad) represent the angular resolution per Y-coordinatevalue 1 of camera 10, then Equation 1 below will hold.

dy·Δθ(rad)=(h−Δh1)/(d2+Δd)  Equation 1

The vehicle-to-vehicle distance (second distance d2) can be calculatedfrom Equation (1). Data indicating the second distance d2 calculated inthe second distance calculation circuit 29 is input to the distancedecision circuit 15.

FIG. 8 is one example of the target-vehicle image 1 detected from thecaptured image 30A.

The detected target-vehicle image 1 contains an image 7 of a tire. Ifthe target vehicle is detected as the target-vehicle image 1 inclusiveof the tire image 7, then a detection frame 41 at this time will belower than the detection frame 40 that results when the tire image 7 isnot contained in the target-vehicle image. As a consequence, theY-coordinate position yb of the lower edge of the rear end oftarget-vehicle image 1 detected as set forth above will be lower by anamount commensurate with the tire image 7. As shown in FIG. 1,therefore, the second distance d2 will be calculated taking intoconsideration the portion equivalent to the height of the tire image 7of the target vehicle 1. The second distance d2 is calculated inaccordance with Equation 2 below.

dy·Δθ(rad)=h/(d2+Δd)  Equation 2

If the detected target-vehicle image 1 does not contain the tire image7, the lower-edge position is corrected by the lower edge positioncorrection circuit 13 in such a manner that the second distance d2 willbe calculated based upon Equation 2. The determination as to whether thetarget-vehicle image 1 does not contain the tire image 7 may be made byverifying whether the tire image 7 is not included in the lower portionof the detection frame 40 or 41, or by verifying whether the tire image7 is not included beneath the frame 40 or 41 on the outer side thereof.

FIG. 9 is one example of the target-vehicle image 1 detected from thecaptured image 30A.

In the foregoing embodiment, the second distance d2 is calculatedutilizing the Y-coordinate position yb of the lower edge oftarget-vehicle image 1. However, the second distance d2 can becalculated also by utilizing a Y-coordinate position yu of the upperedge of the rear end of target-vehicle image 1.

The rear end of the target-vehicle image 1 is detected and is enclosedby a detection frame 42. The detection frame 42 encloses thetarget-vehicle image 1 so as to exclude the rear window of the targetvehicle. The upper edge of the detection frame 42 is the Y-coordinateposition yu.

FIG. 10, which corresponds to FIG. 1, is a side view showing therelationship between one's own vehicle 2 and the target vehicle 1.

Since the upper portion of the rear end of target vehicle 1 is at aposition having a height Δh2 from the road 3, the second distance d2 iscalculated in accordance with Equation 3.

dy·Δθ(rad)=(h−Δh2)/(d2+Δd)  Equation 3

A more accurate second distance d2 can be calculated by adopting theaverage distance of second distance d2 (third distance) calculated fromEquation 1 or 2 and second distance d2 (fourth distance) calculated fromEquation 3 as the second distance d2. In a case where the seconddistance d2 is calculated in accordance with Equation 3, the upper-edgeposition yu would be decided in the above-described lower edge positiondecision circuit 26, and data representing the upper-edge position yu,the lower-edge position yb and the vanishing-point position ye would beinput to the second distance calculation circuit 29.

The data representing the second distance d2 thus calculated is input tothe distance decision circuit 15. Besides the data representing thefirst distance d1 calculated in the first distance calculation circuit14, data representing the position of the vehicle image detected in thevehicle image detection circuit 11 and data indicating the position ofthe vanishing point detected in the vanishing point detection circuit 28also are input to the distance decision circuit 15, as mentioned above.

The distance d to the target vehicle 1 is calculated in the distancedecision circuit 15 in accordance with Equation 4 below. It should benoted that in Equation 4, d1 is the first distance calculated in thefirst distance calculation circuit 14, d2 is the second distancecalculated in the second distance calculation circuit 29, and k is aweighting coefficient that varies in accordance with the distance fromthe target-vehicle image 1 to the vanishing point Pv.

d=(1−k)×d1+k×d2  Equation 4

FIG. 11 illustrates the relationship between distance from thetarget-vehicle image 1 to the vanishing point Pv and the weightingcoefficient k.

If the distance from the target-vehicle image 1 to the vanishing pointPv is short, the weighting coefficient k is k1. Similarly, if thedistance to the vanishing point Pv is neither short nor long but medium,then the weighting coefficient is k2. If the distance to the vanishingpoint Pv is long, the weighting coefficient is k3. It should be notedthat 0<k1<k2<k3<1 holds. Accordingly, the shorter the distance from thetarget-vehicle image 1 to the vanishing point Pv, the less thevehicle-to-vehicle distance d decided based upon Equation 4 isinfluenced by the second distance d2 calculated based upon the vanishingpoint Pv. The closer the target-vehicle image 1 is to the vanishingpoint Pv, the more any error in calculation of the second distance d2 isinfluenced by any error in detection of the vanishing point Pv. However,since the closer the target-vehicle image 1 is to the vanishing pointPv, the more the influence of the second distance d2 upon thevehicle-to-vehicle distance d can be reduced, the influence that anyerror in detection of the vanishing point Pv has upon thevehicle-to-vehicle distance d can be diminished. The vehicle-to-vehicledistance d can thus be calculated comparatively accurately.

When data representing the vehicle-to-vehicle distance d is decided inthe distance decision circuit 15 every unit time, the data representingthe vehicle-to-vehicle distance d is input to a collision timecalculation circuit 16, time measurement circuit 17 and driving displaycircuit 25.

The time measurement circuit 17 checks to determine whether a state inwhich the vehicle-to-vehicle distance is less than a hazardous distanceat which the danger of a collision will occur has continued to a certainextent. If this state where the vehicle-to-vehicle distance is less thanthe hazardous distance continues to a certain extent, data indicative ofthis fact is applied from the time measurement circuit 17 to a warningunit 18. The warning unit 18 issues a warning to the driver of vehicle 2in the form of a warning tone or warning display, etc. Further, arecording control unit 21 is controlled so that the image data capturedby the camera 10 is recorded in a recording unit 22 as moving image dataindicative of hazardous driving and as continuous still image data.

When the data representing the vehicle-to-vehicle distance is applied tothe collision time calculation circuit 16 every unit time, the collisiontime calculation circuit 16 predicts the time at which thevehicle-to-vehicle distance will become zero. If the collisionprediction time reaches a predetermined time, the collision timecalculation circuit 16 applies data to this effect to the warning unit18. The warning unit 18 issues a warning in the manner described above.Further, an engine control circuit 19 is controlled in such a mannerthat a collision will not occur, and the speed of one's own vehicle 2 isthus diminished.

Further, the speed of one's own vehicle 2 is detected by a speeddetection circuit 24. Data indicating the detected speed is applied tothe driving display circuit 25.

The driving display circuit 25 displays a graph indicative of a driver'sdriving tendency, which indicates the relationship between the travelingspeed of one's own vehicle 2 and vehicle-to-vehicle distance.

FIGS. 12 and 13 are examples of displays of driving tendency. In bothexamples the horizontal axis is a plot of traveling speed and thevertical axis a plot of vehicle-to-vehicle distance.

In FIGS. 12 and 13, a graph G indicates a relationship, which isconsidered safe for driving, between traveling speed andvehicle-to-vehicle distance.

The relationship between traveling speed and vehicle-to-vehicle distanceillustrated by graph G changes depending upon the traveling speed. Iftraveling speed is low, a comparatively short vehicle-to-vehicledistance is acceptable. If traveling speed is medium, however, then acomparatively greater vehicle-to-vehicle distance is necessary. Whentraveling speed is high, a long vehicle-to-vehicle distance isnecessary.

If vehicle-to-vehicle distance is greater than the vehicle-to-vehicledistance indicated by graph G such that the relationship betweenvehicle-to-vehicle distance and traveling speed falls within a region S1indicated by the hatching, then this is indicative of a safe drivingpattern. On the other hand, if the relationship betweenvehicle-to-vehicle distance and traveling speed falls within a region S2so that the vehicle-to-vehicle distance is less than thevehicle-to-vehicle distance indicated by graph G, then this isindicative of a hazardous driving pattern. These patterns are obtainedin conformity with traveling speed.

FIG. 12 illustrates safe driving and hazardous driving tendencies basedupon a scatter diagram.

A number of points 50 indicating the relationship between travelingspeed and vehicle-to-vehicle distance are illustrated as mentionedabove. The driving tendency of the driver can be understood inaccordance with the distribution of the points 50. In cases wheretraveling speed is low in FIG. 12, the driver is engaged insubstantially safe driving, but when traveling speed rises to the mediumlevel, the vehicle-to-vehicle distance shortens and the tendencyindicated is one of hazardous driving. Further, it will be understoodthat when traveling speed is high, the vehicle-to-vehicle distancebecomes long and the tendency indicated is one of safe driving. Forexample, an arrangement may be adopted in which a difference Δ1 betweena required vehicle-to-vehicle distance and the actual vehicle-to-vehicledistance is calculated at a specific traveling speed and the driver isnotified of the existence of this difference Δ1.

FIG. 13 illustrates driver tendency using a bar graph.

The relationship between traveling speed and vehicle-to-vehicle distanceis illustrated by multiple bars 51 to 55 of the bar graph. It will beunderstood that whereas bars 51, 52 which result when traveling speed islow indicate that the necessary vehicle-to-vehicle distance exists, bars53, 54 which result when traveling speed is medium indicate that thevehicle-to-vehicle distance is shorter than the necessaryvehicle-to-vehicle distance and, hence, that driving is hazardous.Further, it will be understood that bar 55 which results when travelingspeed is high indicates that the vehicle-to-vehicle distance is thenecessary vehicle-to-vehicle distance and that driving is comparativelysafe.

The driving tendency of the driver is thus displayed by the drivingdisplay circuit 25. The driver can dedicate himself to safe drivingwhile viewing the display.

Further, an arrangement may be adopted in which data indicating therelationship between calculated traveling speed and vehicle-to-vehicledistance is extracted and the above-described driving display ispresented at the driver's home or office or the like at the conclusionof driving.

In the foregoing embodiment, in the case where the firstvehicle-to-vehicle distance d1 is calculated, the vehicle group of atarget vehicle is decided from the width of a target-vehicle image anddistance to the target vehicle is calculated based upon the vehiclewidth that corresponds to the vehicle group decided. However, anarrangement may be adopted in which the height of the target-vehicleimage is detected, the group of the target vehicle is decided from thedetected height, and the distance to the target vehicle is calculatedbased upon the vehicle height that corresponds to the vehicle groupdecided. In this case, rather than the vehicle width memory 13, usewould be made of a vehicle height memory in which a vehicle height hasbeen stored for every vehicle group.

A license plate differs for every vehicle group such as light-dutyvehicle, standard passenger car and heavy-duty vehicle. The vehicleimage detection circuit 11 may therefore be adapted so as to detect theimage of the license plate of the target vehicle and decide the vehiclegroup of the target vehicle from the detected image of the licenseplate. Further, it may be so arranged that in a case where a vehiclegroup cannot be determined by the vehicle group decision circuit 12, thevehicle group is determined by referring to the image of the licenseplate.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the appended claims.

What is claimed is:
 1. A vehicle-to-vehicle distance calculationapparatus comprising: an imaging control device for controlling acamera, which has been mounted on one's own vehicle, so as to image atarget vehicle present ahead of one's own vehicle; a vehicle groupdecision device for deciding to what group the target vehicle belongsbased upon an image obtained by imaging by the camera; a target vehiclepixel count calculation device for calculating number of width or heightpixels of a target image contained in an image obtained by imaging bythe camera; a first distance calculation device for calculating thedistance to the target vehicle based upon a representative vehicle widthor vehicle height of the vehicle group decided by said vehicle groupdecision device, the number of pixels calculated by the target vehiclepixel count calculation device and the number of width or height pixelsof the image obtained by imaging by the camera; a target-vehicle imagedetection device for detecting a target-vehicle image, which representsa target vehicle, from within the image obtained by imaging by thecamera; a vanishing point detection device for detecting a vanishingpoint from within the image obtained by imaging by the camera; a seconddistance calculation device for calculating the distance to the targetvehicle based upon a position of at least one of an upper edge and loweredge of the target-vehicle image detected by said target-vehicle imagedetection device and position of the vanishing point detected by saidvanishing point detection device; and a distance decision device fordeciding the distance to the target vehicle from the first distancecalculated by said first distance calculation device and the seconddistance calculated by said second distance calculation device uponapplying weighting such that the closer together the position of thetarget-vehicle image detected by said target-vehicle image detectiondevice and position of the vanishing point detected by said vanishingpoint detection device, the smaller the weight of the second distancecalculated by said second distance calculation device becomes.
 2. Theapparatus according to claim 1, further comprising a vehicle memory inwhich representative vehicle widths or vehicle heights have been storedbeforehand on a per-vehicle-group basis; wherein said first distancecalculation device calculates the distance to the target vehicle basedupon a representative vehicle width or vehicle height of the vehiclegroup, which has been decided by said vehicle group decision device,from among the representative vehicle widths or vehicle heights thathave been stored in said vehicle memory, the number of pixels calculatedby said target vehicle pixel count calculation device, and the number ofwidth or height pixels of the image obtained by imaging by the camera.3. The apparatus according to claim 1, wherein said first distancecalculation device includes: a third distance calculation device forcalculating a third distance to the target vehicle based upon the upperedge of the target-vehicle image detected by said target-vehicle imagedetection device and the position of the vanishing point detected bysaid vanishing point detection device; and a fourth distance calculationdevice for calculating a fourth distance to the target vehicle basedupon the lower edge of the target-vehicle image detected by saidtarget-vehicle image detection device and the position of the vanishingpoint detected by said vanishing point detection device; the distance tothe target vehicle being calculated based upon the third distancecalculated by said third distance calculation device and the fourthdistance calculated by said fourth distance calculation device.
 4. Theapparatus according to claim 1, further comprising: a determinationdevice for determining whether a tire of the target vehicle is absentbelow the lower edge of the target vehicle detected by saidtarget-vehicle image detection device; and a correction device forcorrecting the position of the lower edge of the target vehicle, whichhas been detected by said target-vehicle image detection device, inresponse to a determination by said determination device that a tire isabsent; wherein said second distance calculation device calculates thedistance to the target vehicle using the position of the lower edgecorrected by said correction device.
 5. A vehicle-to-vehicle distancecalculation method comprising the steps of: controlling a camera, whichhas been mounted on one's own vehicle, so as to image a target vehiclepresent ahead of one's own vehicle; deciding to what group the targetvehicle belongs based upon an image obtained by imaging by the camera;calculating number of width or height pixels of a target image containedin an image obtained by imaging by the camera; calculating the distanceto the target vehicle based upon a representative vehicle width orvehicle height of the vehicle group decided, the number of pixelscalculated and the number of width or height pixels of the imageobtained by imaging by the camera; detecting a target-vehicle image,which represents a target vehicle, from within the image obtained byimaging by the camera; detecting a vanishing point from within the imageobtained by imaging by the camera; calculating the distance to thetarget vehicle based upon a position of at least one of an upper edgeand lower edge of the target-vehicle image detected and position of thevanishing point detected; and deciding the distance to the targetvehicle from the first distance calculated and the second distancecalculated upon applying weighting such that the closer together theposition of the target-vehicle image detected and position of thevanishing point detected, the smaller the weight of the second distancecalculated becomes.